Interpretive Summary: Imported fire ants were introduced in the United states in the early 1900s and currently infest more than 320 million acres in the southern United States, PUerto Rico, California, Arizona, and New Mexico. They cause numerous problems for humans, domestic animals, agriculture, electrical structures, and wildlife. Fire ants cause nearly $6 billion annually in losses and control cost. Fire ant populations in the United States are 5-10 times higher than in South America and the absence of their natural enemies is the more likely explanation. Phorid flies are promising fire ant biocontrol agents because primarily they reduce the foraging behavior of fire ants, facilitating the competition of other ants that might be excluded from food sources in fire ant territories. At least 22 species of Pseudacteon flies have been found attacking fire ants in South America. However, the distribution and abundance of Pseudacteon species in souther South America, which includes several unexplored areas. This study indicated that Pseudacteon species are common in souther South America and show similar abundance and richness of species in the different regions surveyed. Some Pseudacteon species were widley distributed on several host in this region. These results will facilitate the fly exportation to quarantine facilities in the United states to be used against imported fire ants.

Technical Abstract:
The distribution and abundance of fire ant decapitating flies (Diptera: Phoridae: Pseudacteon Coquillett) were studied in three regions of southern South America primarily from September 2002 to September 2004. A total of 2,421 flies belonging to 14 Pseudacteon species were found at 51% of the 662 fire ant mounds examined at 125 collecting sites. Flies occurred in a wide variety of habitats at altitudes from sea level to 2,280 m. Pseudacteon obtusus Borgmeier (large form) was found at the highest altitude and at the most western longitude. Flies were active between 16 degrees Celsius and 37 degrees Celsius, 20% and 90% relative humidity, and 0 and 11.6 km/h wind speed. Pseudacteon curvatus Borgmeier showed the highest abundance and one of the broadest geographical distributions. Pseudacteon tricuspis Borgmeier, P. litoralis Borgmeier, the large from of P. obtusus, P. nudicornis Borgmeier, and P. nocens Borgmeier were also widely distributed. These species appear to be most generalized within saevissima-group. Pseudacteon solenopsidis Schmitz was only collected attacking isolated workers. A new Pseudacteon species was discovered in northwestern Argentina. Seven fly species were reported for the first time on a new fire ant host in this region. Pseudacteon cultellatus Borgmeier was found for the first time on S. invicta Buren in Corrientes province in northeastern Argentina, where up to nine fly species have been found to co-occur. Males of P. tricuspis and P. obtusus were the only males normally attracted to disturbed fire ant colonies.